Thanks, but I'm not talking about an umbrella but a softbox
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OliverSep 7 '12 at 9:20

Then the ribs are even less visible. And the flash inside does not leave a shaddow because the light bounces off the silver lining. The light will hit close enough to the center to resolve your worries.
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Pat FarrellSep 7 '12 at 17:31

I've seen one yesterday, and as a complement of my question, I can tell that there is a real problem with this kind of softbox. If the flash is inside the softbox, you can't incline it much, really not. It has to be almost always vertical. Don't even think of putting it at 45° with a single tripod because the base of the tube cannot go through the border of the box. If you have a mobile arm on your tripod, it's another affair.

This depends a lot on the specifics of the softbox, but yes, as delivered from Westcott, my 43" Octa has a limited range of angles if you insist on putting the stand through the zippered adjustment slot. If you want more adjustment, just spend $15 on a paulcbuff.com/mba.php Then you can adjust it anyway you want. Plus it balances better.
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Pat FarrellSep 9 '12 at 23:44

@PatFarrell:Ok, but as far as I can see, using that kind of arm would result in a worse angle range.
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OliverSep 10 '12 at 8:27

The PCB arm gives you flexiblilty. With it, you can point the softbox at 45 or even 60 degress down, so its coming from above the user. With the stock arm, you can only point it down 20 or so degrees.
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Pat FarrellSep 10 '12 at 16:21

You can see that with the bare flash, the internal structure is visible when photographing the softbox directly, although really the effect is rather small and I don't think likely to have a very significant effect on the results. The important thing, though, is that if you push a Sto-Fen style "bare bulb" diffuser on the flash, this causes enough light to bounce around internally that there is no almost sign of the internal structure even when photographing directly.